Many vehicles comprise a valve system adapted for piloting selectively, either simultaneous braking of the wheels (such as four wheels) belonging to a front axle and to a rear axle when connected pedals are activated or individual braking of the left or right wheel of a single axle, typically the rear axle, in case disconnected pedals are activated.
The enclosed FIG. 1 illustrated an embodiment of such a valve system known from the state of the art. This system is known as a “dual pedal power braking steering valve.”
On FIG. 1 the respective outputs correspond to the following:                A1 corresponds to a line adapted to be connected to a first source of fluid under pressure, such as a pump or a reservoir of fluid under pressure, for braking simultaneously or selectively the left and right wheels of a rear axle,        A2 corresponds to a line adapted to be connected to a second source of fluid under pressure, such as a pump or a reservoir of fluid under pressure, for braking the wheels of a front axle,        T corresponds to a line adapted to be connected to a tank or drain without pressure,        F1 corresponds to a line adapted to be connected to the braking system of the right wheel of a vehicle rear axle,        F2 corresponds to a line adapted to be connected to the braking system of the left wheel of a vehicle rear axle,        PTAV corresponds to a line adapted to be connected to the braking system of the wheels of a vehicle front axle and        FR corresponds to a line adapted to be connected to the braking system of a trailer.        
The hydraulic circuit illustrated on FIG. 1 comprises 3 pedals: a main braking pedal 10, a right braking pedal 12 and a left braking pedal 14, as well as four control valves: a main rear valve 20, a main front valve 21, an auxiliary right rear valve 22 and an auxiliary left rear valve 24.
When none of the 3 pedals 10, 12 and 14 is activated, all the lines F1, F2 and PTAV are connected to the tank linked to the line T. No braking effect is operated.
When only the right pedal 12 is activated together with the main pedal 10, the main rear valve 20 and the auxiliary right rear valve 22 are switched. The line F1 is connected with the supplying line A1. A braking pressure is applied on the braking system of the right rear wheel.
Symmetrically when only the left pedal 14 is activated together with the main pedal 10, the main rear valve 20 and the auxiliary left rear valve 24 are switched. The line F2 is connected with the supplying line A1. A braking pressure is applied on the braking system of the left rear wheel.
When both the right pedal 12 and the left pedal 14 are activated together with the main pedal 10, all the valves 20, 21, 22 and 24 are switched. The lines F1 and F2 are connected with the supplying line A1 and the PTAV line is connected to the supplying line A2. A braking pressure is applied on the braking system of the right rear wheel and the left rear wheel from the supplying line A1 and simultaneously a braking pressure is applied on the braking system of the front axle from the supplying line A2.
Although such dual pedal power braking steering valve have been manufactured and used extensively in the past, such valves have some drawbacks.
Particularly in some applications, such as agricultural and forestry machines, it appears necessary to provide similar braking on both sides of the vehicle when the vehicle is moved at a velocity exceeding a specific threshold, such as for example 40 km/h. In other words it appears necessary to disengage the steering function when the vehicle velocity is high.
Document US 2009/250997 describes another braking circuit designed to control respectively the braking of a front axle and the braking of a rear axle.
According to this document, when a valve with reference symbol 516 in its FIG. 4 is activated, pressing on one pedal causes the actuation of the brakes of the front axle, while pressing on the other pedal causes the actuation of the rear brakes, left and right. Document US 2009/250997 furthermore provides means designed to manage fault detection in the circuit.